Metal complexes have been used in many plating schemes. Among the complexes are metal, salt/phosphine complexes derived from a nonorganometallic salt of a heavy metal and a triorganophosphine as disclosed in U.S. Pat. Nos. 3,438,805 and 3,625,755. Such complexes are useful in chemical or electroless plating.
U.S. Pat. Nos. 3,700,693 and 3,817,784 are directed to the preparation of certain fluoroorganocopper compounds which are soluble in aprotic solvents and which are useful as coating compositions that are to be adhered to substrates and subsequently thermally decomposed.
U.S. Pat. No. 3,933,878 is directed to Cu(I) plating solutions which may be triorganophosphine complexes with Cu(I) salts.
Australian Patent No. 145,054 discloses compositions for chemical or electroless plating of copper, silver, gold, and other metals from a bath containing a salt of the metal and 2,4-pentanedione.
European Published Patent application No. 0297348 is directed to methods for chemical vapor deposition of copper, silver, and gold using certain cyclopentadienyl metal complexes.
U.S. Pat. No. 3,356,527 is directed to the use of trifluoro- and hexafluoroacetylacetonates of Metal(II) hydrates, halides, alkyl, aryl, hydroxyl and nitro-compounds (e.g., copper(II), nickel, cobalt(II), and lead) in chemical vapor deposition processes. Such processes require the use of a carrier/reducing gas, namely, hydrogen, hydrazine, carbon monoxide or the like.
Oehr and Suhr, Appl. Phys. A45,151-54(1988) discloses the preparation and use of copper(II), bis-hexafluoroacetylacetonate in the chemical vapor deposition of thin copper films. The process requires the use of hydrogen gas in order to obtain metal films deposited.
Houle et al., Appl. Phys. Lett. 46, Jan 1985 204 et seq., describes laser induced chemical vapor deposition of copper using a volatile copper coordination complex. The source of copper for these studies was bis-(1,1,1,5,5,5-hexafluoro-2,4-pentanedionate) copper(II).
Thermal CVD of copper has also been reported from other precursors such as copper chloride (CuCl.sub.2) and copper acetylacetonates (Cu(C.sub.5 H.sub.7 O.sub.2).sub.2). The copper chloride system requires the addition of hydrogen as a reducing agent and operates at temperatures of 400-1200.degree. C. The products of the reaction are copper and hydrogen chloride (HCl), and a reasonable mechanism for this reaction probably involves disassociative chemisorption of both reactants followed by a surface reaction to make HCl which then desorbs. When copper is deposited from copper(II) bisacetylacetonate, such process may or may not include hydrogen reactants or carrier gases. Generally deposition of copper from these sources results in copper films of lower quality (i.e., lower purity, higher resistivity).
While the prior art generally recognizes the desirability of chemical vapor deposition of metals such as copper, precursors and techniques previously tried have not been successful for the reasons such as those set forth as above. In particular, the prior techniques require either unrealistically high processing temperatures or produce films which are contaminated with carbon and/or oxygen. The use of the chloride precursors in particular has required high processing temperatures while the use of acetylacetonate precursors has led to the deposition of films with high levels of carbon and/or oxygen.
Accordingly, it is a primary object of the present invention to provide an improved technique for CVD of copper and other +1 oxidation state metals wherein films of high quality can be deposited at relatively low temperatures.
It is another object of this invention to provide improved CVD processes for the deposition of Cu, Ag, Rh and Ir films of high quality and good surface morphology.
Low temperature requirements for the deposition of transition metals are most easily met by the decomposition of organometallic precursors. However, for certain metals such as copper, this is made difficult by the instability of organocopper compounds and their tendency to form nonvolatile oligomers and polymers . For example, binary alkyl copper complexes undergo autocatalytic decomposition to alkanes or alkenes and copper metal at temperatures too low for the compounds to be practically useful. This instability decreases the concentration of precursor that reacts at the heated substrate and large amounts of reactants will be lost. In the case of binary aryl copper complexes, these materials are more stable, but their oligomeric structure lowers their volatility to the point that decomposition occurs before transport. Again, this means that insufficient quantities of reactants will be delivered to the substrate during metal film growth.
It is another object of this invention to provide a class of precursor compounds which can be used in low temperature thermal and laser induced chemical vapor deposition processes without the problems described in the previous paragraph.
The invention broadly relates to the use of certain (+1) metal coordination complexes for the CVD of substantially pure metal, such as copper, silver, rhodium, or iridium, onto a substrate where the process is a thermally driven or laser induced CVD process. In the course of such process, the oxidation state of the metal goes to (0). The high quality metal complexes include a central metal atom in the (+1) oxidation state such as Cu(I), Ag(I), Rh(I) and Ir(I). Generally, the substrate upon which deposition is desired is heated in order to provide an activated or energized site for the decomposition reaction to occur. In the practice of the present invention, any type of CVD apparatus in which the substrate is heated and/or in which the gaseous precursor is heated in the immediate vicinity of the target substrate may be used with the precursors of the invention. These apparatus include standard thermal reactors, such as cold wall, hot substrate reactors, as well as laser or radiation beam reactors where a laser beam is used to heat the substrate and/or the precursor in the vicinity of the substrate. In the art, it is common to refer to these apparatus and the processes separately so that thermal CVD implies the use of a thermal reactor and laser CVD implies the use of a laser energy source. However, in laser CVD, the laser may provide energy in the ultraviolet wavelength region to alter the mode of precursor decomposition (i.e., electronic wavelength excitation). A laser or plasma may also be used separately or in conjunction with the thermal CVD techniques.
In a suitable thermal CVD process, the reaction vessel is usually a cold wall, hot substrate apparatus in which vapors of the precursor metal complex travel to the heated substrate and decompose on its surface resulting in a pure metal deposit. There is insufficient thermal activation in other regions of the reactor to facilitate any significant decomposition and/or deposition where it is not desired.
In laser induced CVD processes, laser energy may heat the substrate or it may be focused so as to cause the decomposition of the precursor gas in the immediate vicinity of the substrate. The precursor complexes may be such that they do not absorb laser energy in the vapor phase, but decompose on a laser heated substrate or they may be such that they absorb focused laser energy and dissociate on a nearby substrate before they can recombine or be deactivated in the gas-phase.
The deposition techniques of this invention are based upon the discovery that certain stabilized metal coordination compounds are particularly useful in thermal, plasma and laser-induced CVD as precursors for deposition of copper, silver, rhodium and iridium metals. These complexes are all in the (+1) oxidation state. It has been found that the quality of the films deposited is much improved over the films formed from precursors having a (+2) oxidation state as may be found in the prior art, especially for copper.
For metal deposition in accordance with the present invention, unique precursor complexes have been discovered. These complexes comprise a metal in the (+1) oxidation state coordinated to a beta-diketone and provided with one or more stabilizing ligands. In some instances such stabilizing ligands may be shared by more than one coordination complex. The preferred (+1) metal is selected from the group consisting of Cu(I), Ag(I), Rh(I) and Ir(I). The metal complexes of greatest commercial importance are Cu(I) and Ag(I).
The beta-diketone which is believed to form a 6-membered coordinated ring with the metal is of the form R'COCHRCOR" where CO is carbonyl, where R is H, methyl, ethyl or halogen, and where R' and R" may be the same or different and are selected from the group consisting of C.sub.n H.sub.(2n+1), C.sub.n F.sub.(2n+1), aryl and substituted aryl where n is an integer from 1 to 4. Typical of the substituent groups included in R' and R" are --CH.sub.3, --CF.sub.3, --C.sub.2 H.sub.5, --C.sub.2 F.sub.5, --C.sub.3 H.sub.7, ---C.sub.3 F.sub.7, --C.sub.4 H.sub.9 and --C.sub.4 F.sub.9. The most preferred beta-diketones are acetylacetone (acac), trifluoroacetylacetone (tfacac) and hexafluoroacetylacetone (hfacac).
The stabilizing ligands include alkynes, olefins, dienes, and phosphines and may broadly include alkyl, trifluoroalkyl and halogen substituted ligands. Included in the stabilizing ligands are 1,5-cyclooctadiene (COD), fluoro-1,5-cyclooctadiene, methyl-1,5-cyclooctadiene, dimethyl-1,5-cyclooctadiene, cyclooctene (COE), methylcyclooctene, cyclooctatetraene (COT), substituted cyclooctatetraene, norbornene, norbornadiene, tricyclo[5.2.1.0]-deca-2,6-diene, alkyl-substituted tricyclo- [5.2.1.0]-deca-2,6-diene, 1,4-cyclohexadiene, acetylene, alkyl or halogen substituted acetylenes, carbon monoxide, [4.3.0]bicyclo-nona-3,7-diene and substituted [4.3.0]bicyclo-nona-3,7-dienes, amines and substituted amines, trimethyl phosphines and substituted phosphines. It is believed that the ligands impact stability to the complexed metal (I) structure by pi bonding overlap with the metal p and d orbitals. The metal may impart back bonding to the stabilizing ligand(s) as well.
It is believed that there is a very intricate relation wherein the stabilizing ligands bond to the metal center by pi overlap with the p and d orbitals of the metal. They add stability by occupying coordination sites and through pi bond or back-bonding with the metal. Copper(I) can form both trigonal and square planar coordination complexes. Thus, when COE complexes are formed, the coordination number may be 3 and the structure is trigonal. On the other hand where COD is used, the coordination number is 4 and the structure is most likely square planar. For this reason, we do not rely on any premise that favors one shape or structure over any other, but take the position that stabilization occurs when all of the permitted coordinating sites are filled with the respective ligands.
The vertical film growth achieved via CVD methods enables the filling of high-aspect ratio holes or structures. The CVD process affords conformal growth and the potential to completely fill (without the presence of voids) vias, holes and channels with nearly vertical sidewalls. Thus far, vias, holes or channels with an aspect ratio of 2.6 to 1 have been filled by this CVD process and possess only a seam, analogous to that found for blanket tungsten CVD processes. The use of (COD)Cu(I)(hfacac) and (DMCOD)Cu(I)(hfacac) for the CVD of copper films has demonstrated superior conformality, surface smoothness and vertical hole-filling. The ability to completely fill vertical holes or vias enables the fabrication of multi-layered structures with interconnection from a top layer to those underlying metallurgical layers.